1. Industry Characteristics
Hydrogen storage is a way to store energy that competes with batteries. Both batteries and hydrogen store energy in chemical form and need a device to convert the energy to heat or mechanical work. Fuel cells and combustible engines are two main devices that convert hydrogen into electricity through the combining with oxygen. In automobiles, the gas tank can be replaced with a hydrogen tank and the automobile can still work. A strong incentive exists in the United States and elsewhere, however, to find alternative energy means to power automobiles, due to pollution problems and other obvious problems with using gasoline. Of the alternative energy sources, ethanol is inadequate because ethanol still pollutes. Generating hydrogen (H2) and electricity for batteries may also pollute, but this can be done at a central location and thus can be handled more easily. Furthermore, ethanol still needs 15% gasoline to start the car and warm it up. Regarding batteries, they are not yet efficient enough to power automobiles on their own. An effective way of utilizing hydrogen as a fuel in a car did not exist before the present invention.
The ability to use hydrogen as an alternative fuel source instead of gasoline hinges on the creation of materials that can adequately store hydrogen in ambient settings. Currently, the materials that exist do not store hydrogen in adequate amounts. No one has been able to develop a hydrogen storage device that has met the standards put forth by the U.S. Department of Energy. Therefore, a convenient and economical storage system for hydrogen fuel does not exist.
Hydrogen storage technologies face a number of exemplary challenges: 1) the ability to store enough hydrogen for a vehicle to have a driving range equal to or better than that provided by a tank of gasoline; 2) ability to be sufficiently light and compact so as not to change the efficiency of the vehicle; and 3) ability to be economical to provide motivation for switching from gasoline. One persistent problem that has prevented using hydrogen effectively in vehicles has been low gravimetric density. Once these challenges are overcome, hydrogen fuel technologies could quickly become adopted into the mainstream marketplace. Regarding the amount of Hydrogen that must be absorbed by the material before it is considered to be commercially viable, the United States Department of Energy set this standard at 6% of system weight.
In addition to hydrogen storage capacity, a second important parameter that governs the usefulness of a hydrogen storage material is the speed with which the material can take up and discharge hydrogen. This means that the kinetics governing the hydrogen absorption process also has to be favorable.
2. Theory and Problems in the Art
Although a large number of compounds with high hydrogen content actually exist, these compounds have severe limitations. One category of compounds with high hydrogen content is metal hydrides, which are materials that contain strong chemical bonds (i.e. the Hydrogen is absorbed chemically). While these materials can have large hydrogen content, they do not release hydrogen until taken to elevated temperatures, which is a characteristic that limits their practical utility. Another category of compounds with high hydrogen content are physisorbants, which are materials that absorb hydrogen physically, not chemically. While these materials have the ability to release hydrogen at modest temperatures, they do not have the capacity for high hydrogen content under ambient conditions, as the hydrogen only remains bound to the compound at extremely low temperatures. Therefore, physisorbants also have practical limitations.
Carbon-based materials are attractive for hydrogen storage because of their light mass, abundance, favorable chemistry, and high hydrogen content. The challenge in using these materials, however, lies in preserving the hydrogen entrained in its molecular form while at the same time allowing for a large number of hydrogens to desorb near room temperature. In order to meet this goal, several theoretical groups have focused on applying first principles quantum mechanics to various carbon systems with metal atoms. Such work stems from early observations of Kubas that the interaction energy between a carbon-metal complex and hydrogen suggests near room temperature desorption. (1). As a result of these theories regarding carbon-based materials, carbon nanotubes, buckyballs, metcars, and carbon containing polymers have been theoretically studied. (2). Many of these materials are predicted to absorb hydrogen in excess of 6 wt %. But there have also been counter-suggestions that a clustering of metal atoms would degrade the absorption ability of the carbon-based material. (3) Durgun and collaborators recently proposed that transition metals (such as titanium) in reaction with ethylene should form new complexes that have the potential to absorb as many as 5 hydrogen molecules per transition metal atom (4). These are just theoretical abstractions, and prior to this invention, no one has been able to use these theoretical abstractions to actually create a material that is able to absorb and desorb a substantial amount of hydrogen quickly in ambient conditions.
Table 1 shows various predicted hydrogen uptake values that could theoretically result from mixing different transition metals (and lithium) with various carbon-based complexes. (5) However, yet again, no one has been able to actually create a material that is able to absorb and desorb a substantial amount of hydrogen quickly in ambient conditions.
TABLE 1BindingEnergy(electronMetalPrecursorComplexMax #wt %Volt)/H2ScEthyleneTi2C2H4(H2)1014.50.39ScEthyleneTiC2H4(H2)5120.39TiEthyleneTi2C2H4(H2)1013.90.45TiEthyleneTiC2H4(H2)511.60.45TiCyclobutaneTiC4H4(H2)59.100.33ScCyclobutaneScC4H4(H2)59.30.33ScBenzeneScC6H6(H2)n60.40TiBenzeneTiC6H6(H2)n60.69VBenzeneVC6H6(H2)n4.40.83ScC60Sc12C607.00.35ScC48B12Sc12C48B128.770.35LiC60Li12C60130.075
Many have tried to create various transition metal-ethylene systems for purposes other than hydrogen storage. No one has been able successfully identify a transition metal-ethylene complex of practical use for hydrogen storage until the present inventors. Even more, some have tried to create transition metal-ethylene complexes that simply exhibit a hydrogen uptake value of greater than 0, let alone create a system that matches the theoretical predictions, but all have failed. For example, Lee, Manceron, and Papai found a strongly bound Ti(C2H4) system, and similar H2Ti(C2H2) and HTi(C2H3) systems. (6) However, their IR absorption experiments reported negative results for Ti(C2H4). Ozin produced a Nickel-ethylene system, and Kafafi tested n Iron-ethylene system. Despite the efforts of others in the field to produce a Transition metal-ethylene system, to our knowledge, no experimental reports exist showing that a room temperature hydrogen absorbing material has ever been developed. (7) In other words, despite the proposed theories, no one has been able to actually create a material and demonstrate that it can effectively absorb and desorb a high amount of hydrogen at room temperature.
Results from prior testing of transition metal-ethylene complexes (in both the gas phase and on surfaces) suggest that two types of transition metal ethylene structures can be formed. Spectroscopic studies of transition metals and ethylene co-deposited in a solid argon matrix show a pi complex for several transition metals, including nickel (Ni), palladium (Pd), and titanium (Ti) (8). A similar bonding mechanism has been found on Pt(111) surfaces at low temperatures (9) and vibration studies have found evidence of the structure at room temperature on those surfaces as well (10) The pi structure has also been observed through ground state transition metal atoms reacting with ethylene. (11) In these studies, the transition metal atoms were created through laser ablation and thermalized in a helium gas. Once cooled, the ethylene was injected into the plume. A second type of structure with a sigma bond is known to form. (12) Additionally, solid argon studies show that this is the preferred structure for ethylene bonding to some of the transition metals. (13) The same preferential results were found through some laser ablation studies. (14)
It should be appreciated that all previous uses of transition metals in the context of hydrogen storage in patents have been as catalysts. The transition metals dissociate molecular hydrogen at room temperature, especially Pd, Pt, and Ni. The atomic hydrogen can then be absorbed by whatever material is being used for hydrogen storage. Specifically, U.S. Pat. No. 7,101,530, “Hydrogen storage by reversible hydrogenation of pi-conjugated substrates,” of which is hereby incorporated by reference in its entirety, uses transition metals to dissociate molecular hydrogen into atomic hydrogen, which then bonds to carbon atoms at lower temperatures than the substrates would without transition metals. In contrast, the present invention does not use the transition metal as a catalyst.
In contrast, the present invention implements a transition metal as an actual element of the hydrogen storage material. The transition metal is not a catalyst that aids in dissociating molecular hydrogen to atomic hydrogen.
Many have tried to follow various scientific theories to produce experimental conditions that result in materials that absorb high quantities of hydrogen at room temperature. Everyone, up until the present inventors, has failed in pursuit of this goal.